Transparent ceramics such as transparent alumina articles are used in a wide variety of applications such as discharge lamps, supermarket scanner windows, window plates for furnaces, and military applications such as infrared transmitting windows, missile domes and missile windows.
As surveillance and tactical missions become more complex, there is a need to increase the performance of infrared (IR) systems to provide higher quality and higher resolution imagery. Typically, IR systems on tactical missiles are protected by optically transparent windows or domes, which are exposed to a broad range of environmental and operating conditions. The increasing sensor performance requires commensurate improvements in window performance, so that the window does not limit the imaging capability of the sensors. Missile domes are one of the most demanding applications for ceramics. Severe aero-thermal heating occurs as the missile accelerates to its programmed velocity, which necessitates the use of a material with excellent thermal shock resistance. Good thermal shock resistance is a function of the material's intrinsic physical properties and the extrinsic property of high strength coupled with high Weibull modulus—a combination that avoids premature dome failure. All of these physical properties must accompany a ceramic that is transparent over a broad range of wavelengths. The domes, therefore, require a wide band gap ceramic material in either the single crystal or polycrystalline form.
Water droplet impact damage is another consideration for materials exposed to the speeds and altitudes associated with supersonic flight. In addition, abrasion erosion due to sand particles can be a significant problem. The use of ceramics as windows and domes in IR missile systems requires extended service life without degradation of performance, more robust window and dome survivability (e.g., increased scratch resistance, strength, and thermal shock resistance), and low-cost manufacturing processes.
Historically, IR transmitting windows and domes have been fabricated from single crystal and large grain (>10 μm) ceramics including, for example, MgF2, MgAl2O4, AlON and single crystal Al2O3 (sapphire). As noted above, thermal shock resistance is an important consideration, and due largely to its intrinsic properties of high thermal conductivity and low thermal expansion, alumina has higher thermal shock resistance than other candidate dome materials.
While all of these ceramics have been used successfully in this demanding application, each has its limitations in terms of optical and mechanical properties and price/performance trade-offs. Single crystal materials can be expensive and time-consuming to manufacture and machine into the appropriate shapes, and large grain polycrystalline materials often do not have adequate mechanical and thermomechanical properties to meet the increasing demands of hypersonic flight.
As surveillance and tactical missions become more vital and missile speeds increase, there is a need to increase the performance of infrared (IR) systems to provide higher quality imagery. Increasing missile velocities coupled with higher sensor performance requires commensurate improvements in window and dome performance, including hemispherical and ogive (aerodynamic) shapes. An ogive shape enables some combination of increased range, speed, and payload because of reduced drag. The ogive shape also offers improved rain impact resistance and sand erosion resistance and a greater unvignetted field of view. A method that will allow near-net shape processing of an aerodynamic dome using a material that has the benefits of sapphire is desired.
High-density, large grain-sized PCA material, routinely manufactured commercially for lamp envelopes and orthodontia brackets, is not suitable for dome applications since it is translucent due to birefringent scattering of light as it traverses through the many grain boundaries. This intrinsic property results from the hexagonal crystal structure of alumina. The objectionable birefringence is eliminated as the grain size approaches the wavelength of light.
Thus, there is a need to provide novel transparent polycrystalline alumina articles exhibiting improved mechanical and optical properties, particularly for use in missile domes and windows, as well as a need for improved processes for forming such articles. There is a further need to provide such articles that do not exhibit absorption peaks in the transmittance range required. For applications requiring transmittance in the mid-wave infrared (MWIR) the articles should have substantially no absorption peaks from about 2000 nm up to about 5000 nm.